In various rotating machines, a rotor or rotating member is closely confined within a housing or casing and it is imperative that the gap or distance between the casing and the rotating member, referred to as running clearance, be maintained within predetermined limits for safe and effective operation of the machine. One example of such a machine, and one to which this invention is particularly applicable, is a hot gas turbine engine such as an aircraft gas turbine engine. In such an engine, a turbine wheel or rotor having a circumferential row of spaced apart vanes or blades extending therefrom is closely confined within an encircling housing or casing to define a hot gas flow path transversely through the row of blades. Reaction of the blades to the hot gas flow causes rotation of the turbine wheel and appropriate power generation. Any loss of turbine blade reaction from hot gas leakage or bypass through the running clearance space, instead of between turbine blades, represents a potential power loss. Preservation of a minimum clearance gap during engine operation assists in avoiding significant rotational contact of the blades with the encircling casing which may rapidly lead to failure of engine components as well as the engine as an effective power plant. For these reasons, it has become a practice to measure the running clearance of a turbine wheel during its operation and to have a continuous measuring or monitoring system for the running clearance during certain predetermined operations of the turbine. Various operating characteristics of a hot gas turbine engine provide significant difficulties to the use of many known gap and distance measuring devices, particularly those utilizing actual contact with a moving member. For example, the environment of the high speed turbine blades is hostile to measuring devices, reaching extreme temperatures in the range of 1200.degree. F. to 1800.degree. F. In the presence of a hot, contaminating, and corrosive gas stream. This extreme temperature range causes significant differential expansion of various component parts which affects not only any associated measuring means, but also the running clearance gap or distance being measured.
Because of the high speed of the turbine wheel, the described hostile environment, and the openness or spacing of the blades on the rotor, measuring devices or systems requiring contact with the rotor blades have been avoided. With respect to non-contact measuring means, various electrical capacitance systems have been developed to measure the running clearance of hot gas turbine wheels and other bladed wheels or rotors.
In these prior electrical capacitance systems, a probe member with a sensor end thereon is inserted in an appropriate aperture in a turbine rotor housing, for example, so that the sensor end of the probe is exposed to the passing tips of the turbine blades. The sensor end of the probe adjacent the moving blades is fitted with an electrical capacitor electrode which may be positioned in substantially coplanar relationship with the inner surface of the closely confining casing or housing around the turbine wheel. In this position the probe electrode represents one side of the running clearance gap. The tip surface of a turbine blade, at electrical ground potential is gainfully employed as an opposite capacitor electrode, and the other side of the running clearance gap. A change in the clearance gap is a change in the distance between capacitor electrodes and a change in electrical capacitance therebetween. A variance or change in electrical capacitance, by an increase or decrease in the clearance gap from a predetermined value is measured and correlated by appropriate electrical circuitry to indicate the dimensional change in the distance between the tip surface of a turbine blade, and the closely encircling housing represented by the probe electrode.
As previously described, the probe member, and particularly the sensor electrode part thereof, is positioned in a very hostle environment of high temperatures in the presence of contaminating hot combustion gases from the combustion system of the engine, conditions which contribute to early probe deterioration resulting in, for example, a decrease in sensitivity and accuracy. As a consequence, continuing efforts are expended to provide electrical capacitance probes which are more highly resistant to temperature extremes and contamination, and which have increased sensitivity, accuracy and stability.